First work out how long it will take to descend to the flight level shown:
The aircraft will descent 7,000ft at 800fpm so it will take 8 min 45 sec.
Now you have to work out how fat it'll travel in 8min45sec (or 8.75)
So 540 (speed) / 60(minutes) = 9miles per minute
The aircraft will travel 9 miles across the ground every minute.
so 9 x 8.75= 78.75 nautical miles (nm)
remember "distance over speed times time"
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s x t